More than 10 different classes of therapeutic drugs produce serious weight gain in 40-50% of the patients treated with them, thereby contributing to the worldwide epidemic of obesity. Drug-induced weight gain increases a patient's risk of cardiovascular disease, stroke, and diabetes, and often results in non- compliance with treatment. How these drugs induce weight gain is not known. The major objective of this project is to develop C. elegans as a model system for identifying the genes and pathways involved in drug- induced lipid accumulation. Preliminary studies showed that several classes of drug that cause weight gain in patients produced lipid accumulation and delayed maturation in C. elegans. Furthermore, tph-1 mutants treated with clozapine, an antipsychotic drug with weight gain liability, accumulated lipid and arrested without reaching sexual maturity. These studies will examine the hypothesis that drugs associated with weight gain in patients elicit a metabolic shift in C. elegans that leads to lipid accumulation and delayed reproductive maturation. The Specific Aims are to: (1) evaluate drugs that cause weight gain in patients for induction of lipid storage in C. elegans, (2) employ RNAi to screen for possible genes involved in drug-induced lipid accumulation, and (3) use drug-induced metabolic shift (and arrest) as a screen to select drug-resistant mutants. Lipid accumulation will be detected by fluorescence microscopy of animals stained with Nile red (a lipophilic reporter dye), and secondarily with a starvation assay and Sudan black staining. Drugs that produce clinical weight gain will be compared to weight-neutral drugs for their ability to increase lipid accumulation. Analysis of several simple behaviors, including pharyngeal pumping, locomotion, feeding, and defecation, will rule out non-specific effects of the drugs. Potential targets of the drugs will be identified by determining whether RNAi knockdown of 224 genes that normally produces a lean phenotype in C. elegans also inhibits drug-induced accumulation of Nile red. A sensitized selection has been devised based on arrest of maturation of tph-1 animals exposed to clozapine. This assay will be used to select drug-resistant mutants that reach sexual maturity despite growth in the presence of drug. This research may ultimately provide methods for identifying genes and pathways involved in the induction of weight gain by drugs or other substances.